1. Field of the Invention
The invention disclosed in the present specification relates to a thin film transistor using a crystal silicon film and a fabrication method thereof. The invention also relates to a device utilizing such a thin film transistor and a fabricating method thereof.
2. Description of Related Art
Hitherto, there has been known a thin film transistor (hereinafter referred to as TFT) using an amorphous silicon film. It is mainly used for constructing an active matrix circuit of an active matrix liquid crystal display.
However, the TFT using the amorphous silicon film has had a drawback that its operation speed is slow and that a P-channel type TFT cannot be put into practical use. Then, due to such problems, it was unable to fabricate an active matrix liquid crystal display to which a peripheral driving circuit is integrated or to construct various integrated circuits using the TFT.
As a solution of this problem, there has been known a scheme of using a crystal silicon film. The crystal silicon film may be fabricated by heating or by irradiating laser light.
However, the heating method has had a problem that a glass substrate cannot be used because it requires a high temperature process of 900xc2x0 C. or more. Considering that the main field of application of the TFT is a liquid crystal display, it is a preferential subject to be able to use the glass substrate as the substrate.
Meanwhile, although the method of irradiating laser light can realize a process through which no thermal damage is given to the substrate, it is not satisfactory in terms of the crystal uniformity and reproducibility and of the degree of crystallization.
As one of solutions of such problems, there has been a method of promoting the crystallization by using a predetermined metal element, i.e. the inventive method which the present applicants have proposed. According to this method, a crystal silicon film is obtained by introducing metal element typified by nickel to the amorphous silicon film and by implementing a heat treatment. This method allows the crystal silicon film having good crystallinity to be obtained while implementing the heat treatment below about 600xc2x0 C. which permits to use the glass substrate.
However, because the nickel element remains within the crystal silicon film, the characteristic of the TFT fabricated by using the crystal silicon film is influenced adversely. In concrete, it causes problems that the characteristic changes as time elapses, thus degrading the reliability.
Accordingly, it is an object of the invention disclosed in the present specification to provide a technology for suppressing the metal element from adversely influencing the characteristic of the TFT fabricated by using the crystal silicon film obtained by utilizing the metal element which promotes the crystallization of silicon.
One of the invention disclosed in the present specification features a semiconductor device comprising an active layer having a crystal structure in which crystal growth has proceeded from the whole periphery and a semiconductor device utilizing a semiconductor element having such structure.
The structure as described above may be obtained by growing crystal by diffusing nickel 105 from a periphery 201 of a pattern which turns out to be an active layer as shown in FIG. 1 and by causing phosphorus doped to a region 108 outside of the pattern to getter nickel element as shown in FIG. 2.
The crystal structure continues in the direction 105 in FIG. 1 and in the direction 110 in FIG. 2 and a crystal boundary extends along that direction. It may be confirmed by observing through an optical microscope or by TEM (transmission type electronic microscope).
Another scheme of the invention pertains to a fabrication method of a semiconductor device, comprising steps of diffusing metal element promoting crystallization of silicon from the whole periphery of a predetermined region of an amorphous silicon film within the region to grow crystal and of removing the metal element out of the region by following the course reverse to the diffusing course.
This scheme features the steps of growing crystal by diffusing nickel element from the periphery as shown in FIG. 1B-1 and of moving (removing) the nickel element to the periphery as shown in FIG. 2A-1.
The diffusion of the nickel element plays an important role in the crystallization. However, it is not desirable for the nickel element to remain within the crystal silicon film thus obtained. Then, it becomes important how to remove the nickel element effectively.
The course for removing the nickel element to the periphery of the pattern as shown in FIG. 2A-1 is reverse to the course of diffusing the nickel element during the crystallization. Because this course has become a course for moving the nickel element once, its energy level is minimized as a course for moving nickel. That is, it has become a course where the hindrance is least for moving nickel. It is irrelevant to the moving direction of nickel.
Accordingly, the nickel element moves following the most effective course for removing the nickel element as shown in FIG. 2A-1.
Another scheme of the invention pertains to a fabrication method of a semiconductor device, comprising steps of forming a mask on an amorphous silicon film; selectively doping metal element which promotes crystallization of silicon to the amorphous silicon film by utilizing the mask; implementing a heat treatment to grow crystal from the region where the metal element has been introduced to the lower part of the mask in the amorphous silicon film; selectively introducing an element in the XV group to the silicon film by using the mask; implementing a heat treatment to move the metal element to the region where the element in the XV group has been doped; and removing the region where the element in the XV group has been doped.
As the metal element promoting the crystallization of silicon in the present invention, one or a plurality of kinds of elements selected among Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Au, Ge, Pb and In may be utilized. The better effect and the high reproducibility may be obtained when Ni is used in particular.
An element selected among P, As and Ab is utilized as the element of the XV group. The better effect may be obtained when P (phosphorus) is used in particular.
In concrete, a mask 103 made of a silicon oxide film is formed on an amorphous silicon film 102 at first as shown in FIG. 1A. Then, nickel element is held selectively on the surface of the amorphous silicon film 102 by utilizing the mask 103. This process corresponds to the introduction of nickel element.
While there are sputtering, CVD, plasma treatment, ion implantation and the like as methods for introducing nickel element, the method of using a solution is simplest.
Next, a heat treatment is implemented to grow crystal as indicated by arrows (105) in FIG. 1B-1. This crystal growth occurs with the diffusion of nickel element.
Next, phosphorus is introduced as shown in FIG. 1C by means of plasma doping or ion implantation. Phosphorus is doped to a region 108 by using the mask 103 in this step.
Then, another heat treatment is implemented to remove the nickel element from the pattern under the mask 103 by following the course as indicated by arrows 110 reverse to the previous course of diffusing the nickel element in growing the crystal as shown in FIG. 2A-1.
Then, the nickel element is removed by gettering nickel to the region where phosphorus has been doped.
Then, the silicon film is patterned by utilizing the mask 103 again to form a pattern 111.
Thus, the pattern of the active layer which has high crystallinity and from which the influence of the nickel element has been removed may be obtained without increasing masks in particular (i.e. without complicating the process).
The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawings in which like numerals refer to like parts.